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APPLICATION OF MINIMUM-COST OPTIMIZATION TO LIMIT STATE DESIGN OF BRIDGES

Details are given of an optimization method suitable for the solution of non-linear constrained optimization problems of structural design when the computer time required is moderate. The application to generate design aids (feasibility curves, cost sensitivity of fixing parameters and values of the key variables) is demonstrated for A bridge deck in the following areas: (1) for a given span, the designer can use curves to obtain the appropriate combination of beam depth, beam spacing and number of debonding stations for a feasible design. (2) the relative costs for arbitrarily fixing the depth and beam spacing can be compared simply by inspecting the curves. If it is possible to alter the span, information is available to assess the saving achieved by making full use of the casting-bed space. (3) for the detailed design, the prestressing force can be estimated from the curves, thus providing a satisfactory optimum and at the same time simplifying the rest of the checking calculations. (4) as A result of the sensitivity study, the strength of the prestressing strands was found to be cost critical, while the effects of concrete strength and of limiting tensile stress were negligible. Of the various partial safety factors for loads, those related to the limit state of collapse had little effect on the design while factors on serviceability due to dead load and live udl were more Significant. /TRRL/